It is known in the electronics art, particularly the semiconductor art, to incorporate substrate temperature sensing elements to detect increases or decreases in substrate temperature for purposes of protecting the device or circuit from failure from uncontrolled temperature increase.
PN junctions make convenient temperature sensing elements for semiconductor devices. The current through a forward biased PN junction depends on the applied voltage, junction area and junction temperature and, when properly calibrated, can be used as a measure of junction temperature. If the sensing current is small and the thermal conductivity of the substrate material adequate, the temperature at the sensing junction is a reasonable measure of the adjacent substrate temperature. Where monolithic (built into the substrate) PN junctions are used, this is a reasonable approximation in most situations.
However, a difficulty encountered in using substrate PN junctions is that they are electrically connected to the substrate. This makes them difficult to use from a circuit point of view. It is hard to provide multiple PN junctions in series or parallel and, unless elaborate construction or circuit techniques are employed, it is difficult or impossible to electrically isolate them from the device or circuit, on the same substrate, that they are intended to protect.
A partial solution to this problem has been proposed by Y. Tsuzuki et al. (hereafter Tsuzuki), in an article entitled "Self-Thermal Protecting Power MOSFET's", Proceedings of the Power Electronics Specialist's Conference, June 1987, pages 31-37. Tsuzuki utilizes a string of series connected diodes formed in a polysilicon layer on the field oxide of a power MOSFET to prevent the power MOSFET from overheating. Because the polysilicon temperature sensing diodes are electrically isolated from the substrate they can be arranged in a voltage divider circuit controlling the state of a bi-stable latch formed on the same chip. When the temperature of the sensing diodes increases in response to increases in the substrate temperature, the forward voltage drop across the sensing diode string decreases below the toggle point of the latch. When the latch switches, the gate of the power MOSFET coupled thereto is brought to a voltage sufficient to turn the device off so that thermal damage is precluded.
While the arrangement of Tsuzuki overcomes some of the problems in the prior art, it continues to suffer from a number of disadvantages. For example, there is a substantial thermal impedance between Tsuzuki's temperature sensing diodes and the semiconductor substrate the diodes are intended to protect. This is because Tsuzuki's diodes are thermally separated from the substrate by the field oxide. Field oxide (e.g., SiO.sub.2) is approximately one hundred times less thermally conductive than Tsuzuki's silicon substrate. Other things being equal, poly diodes of the type utilized by Tsuzuki will be slower to respond to changes in substrate temperature and will not as accurately track substrate temperature as is desired. Thus, a need continues to exist for improved temperature sensing arrangements that are electrically isolated from but better thermally coupled to the semiconductor substrate they are intended to protect.